The present invention relates to an improved cycle selection technique for use in LORAN-C receivers. Although specifically described in terms of a LORAN-C system, the inventive technique is also generally applicable to any system in which the time between pulsed carrier signals is to be accurately measured or in which a specific carrier cycle in a pulsed carrier transmission is to be accurately tracked.
LORAN-C systems are hyperbolic navigation systems in which transmitters at fixed locations transmit 100 KHz carrier pulses at known pulse repetition rates. A LORAN-C receiver measures the time between reception of such pulses from a master transmitter and each of two slave transmitters. The time between reception of the master pulse and one slave pulse determines a hyperbolic line of position on which the receiver is located. The time between reception of the master pulse and the other slave pulse determines a second hyperbolic line of position on which the receiver is located. The intersection of these two hyperbolic lines accurately defines the position of the receiver. Standard charts for determining position from measured time differences are readily available and in widespread use.
Bandwidth limitations require that the rise time of the transmitted LORAN-C pulse be relatively long, on the order of 50 microseconds. Since the carrier frequency is 100 KHz, the rise time of the pulse extends over approximately five or more carrier cycles. Under such circumstances accurate time measurement cannot be achieved by merely monitoring corresponding amplitude points on the pulse envelope. Therefore, it has become common practice to measure the corresponding points on the received 100 KHz carrier. Specifically, the time of a point on the carrier (for example, a zero crossover) can be determined with much greater accuracy than a point on the much more slowly changing pulse envelope. However, since there are a minimum of ten (and usually more) carrier cycles in each LORAN-C pulse, a cyclic ambiguity exists which can introduce unacceptable errors into the position determination.
There have been a number of prior art approaches to resolving cycle ambiguity in LORAN-C receivers, most of which use an envelope deriver circuit. The envelope deriver circuit sums the received 100 KHz carrier with a version of that carrier which is delayed by 1/2 cycle (i.e., 5 microseconds) and amplified. The sum is a 100 KHz signal which, depending upon the amplification of the delayed signal, can be arranged to exhibit a phase reversal at the termination of a predetermined carrier cycle in the received pulse. This sum signal, called the envelope-derived signal in LORAN-C parlance, is then hard limited and then sampled in order to determine the location of the phase reversal. If each half cycle of the hard limited envelope-derived signal is examined, a series of alternating positive and negative cycles will be found except at the point of phase reversal where two successive half cycles of like polarity exist. Numerous techniques come to mind for finding this point; however, the problem is greatly complicated by the fact that the LORAN-C pulses are more often than not received in a high-noise environment. It is therefore necessary to monitor the carrier cycles for many received pulses and establish some sort of statistical evaluation after numerous samplings have been made. For example, in one prior art technique, after a coarse acquisition procedure is performed in which a tracking pulse is positioned in time coincidence with some zero-crossover in the carrier of each received pulse, four sampling pulses are generated. One sampling pulse is generated three quarters of a carrier cycle (i.e., 7.5 microseconds) before the tracking pulse; a second sampling pulse is located one quarter cycle (i.e., 2.5 microseconds) before the tracking pulse; and third and fourth sampling pulses are located one-quarter and three quarters of a cycle, respectively, after the tracking pulse. These sampling pulses are used as gates to determine when the hard-limited envelope-derived carrier is to be sampled. If the tracking pulse is properly positioned at the point of phase reversal, samples one and four should be high (binary "1") and samples two and three should be low (binary "O"). Four up/down counters are used to register the "1" and "O" levels so that each time a LORAN-C pulse is received, each counter registers an up or down count, depending upon its corresponding sampled level. After some predetermined number of LORAN-C pulses are received, the counts in the counters are examined to see whether they have net up or down counts. If an up-down-down-up net pattern results, the tracking pulse is considered to be on the proper cycle; if other net patterns are found, the tracking pulse is shifted in time by one carrier cycle and the process is repeated.
The cycle selection technique described above works quite well in the presence of strong received signals. However, low signal-to-noise conditions render that technique unreliable. Specifically, noise tends to render one or more of the sampled binary levels erroneous during each received pulse. As a consequence, even when the tracking pulse is positioned on the proper cycle, many of the sample patterns will not show up as 1-0-0-1 but as some other pattern. Therefore the net pattern, which is examined after the specified number of LORAN-C pulses are received, will often indicate incorrect positioning of a properly positioned tracking pulse.
It is therefore an object of the present invention to provide a cycle selection technique for LORAN-C systems which operates reliably in the presence of poor signal-to-noise conditions.
It is another object of the present invention to provide a method for resolving cycle ambiguity in the tracking of pulsed carrier signals.
It is another object of the present invention to provide apparatus for use in a receiver which permits a particular carrier cycle in a pulsed carrier signal to be identified and tracked in a low signal-to-noise environment.